The invention relates to a brake master cylinder for a motor vehicle.
The invention relates more specifically to a brake master cylinder for a motor vehicle, of the type which comprises a substantially axial body inside a bore of which is slideably mounted at least one axial piston which is capable of being actuated by a driver of the vehicle between a rear rest position and a forward braking force application position, and which is returned elastically toward its rear rest position, of the type in which the bore comprises two seals, front and rear, which are interposed between the piston and the bore, the front seal delimiting, in the bore, a rear supply chamber and a front pressure chamber, of the type in which the body comprises a radial supply duct which connects an external hydraulic fluid reservoir to the rear supply chamber and which opens out between the two seals, of the type in which the body comprises a braking circuit supply hole which opens into the front pressure chamber, of the type in which the piston comprises a bore, open to the front, communicating on the one hand with the front pressure chamber and on the other hand with the periphery of said piston by way of at least one hole which, when the piston occupies its rear rest position, is arranged between the two seals so as to open communication between the front pressure chamber and the rear supply chamber and which, when the piston is moved axially forward toward its application position, is capable of passing beyond the front seal in order to isolate the front pressure chamber from the rear supply chamber and thus allow a braking pressure to be established in the front pressure chamber, of the type in which at least each front seal is accommodated in a groove in the body of the master cylinder and comprises three concentric lips with an axial orientation, in particular, a first inner lip of which a free end is arranged in contact with the periphery of the piston, a second central lip of which the free end is arranged, in a rest position, in contact with a front face of the groove, and a third outer lip of which a free end is arranged, in a rest position, in contact with a peripheral bottom face of the groove, the second central lip and the third outer lip being capable, when the front pressure chamber is exposed to a partial vacuum caused by the return of the piston from its forward application position to its rest position or else by the activation of a trajectory control device forming part of the braking circuit, of separating from the front face and from the bottom peripheral face of the groove so as to respectively allow the reservoir to be resupplied by means of the front pressure chamber or else the front pressure chamber to be resupplied by the reservoir.
Many examples of brake master cylinders of this type are known.
In most master cylinders of this type, the piston is generally guided in the bore by way of at least two annular bearing surfaces of the bore, these surfaces being respectively arranged in front of the front seal and to the rear of the real seal.
The bearing surface which is arranged in front of the front seal adjoins the groove which accommodates the front seal and it comprises so-called resupply grooves which are intended to allow brake fluid to flow through these resupply grooves when the front pressure chamber is exposed to a partial vacuum caused by the return of the piston from its forward application position to its rest position or else when an “ESP”-type trajectory control device of the braking circuit is activated and generates a partial vacuum in the front pressure chamber.
The groove which accommodates the front seal generally comprises a straight transverse front face joined to a straight transverse rear face by way of a peripheral bottom face. The second central lip and the third outer lip of the front seal have ends which are intended to adhere respectively to the straight transverse front face and to the peripheral bottom face of the groove.
When the front seal is exposed to a partial vacuum, the ends of the second central lip and of the third outer lip of the front seal respectively separate from the straight transverse front face of the groove and from the peripheral bottom face of the groove so as to respectively allow the reservoir to be resupplied by means of the front pressure chamber or else the front pressure chamber to be resupplied by the reservoir.
What has been found in many cases, however, is that the central lip and the third outer lip of the front seal separate only imperfectly from the front face of the groove and from the peripheral bottom face of the groove. In that case, the brake fluid flow rate is insufficient for the resupply process to proceed correctly.
This is particularly evident when the vehicle is equipped with an “ESP”-type trajectory control device, in which the demand for brake fluid, which is intended to supply one or more of the brake calipers of the vehicle, can be high even when the piston is occupying an application position in the pressure chamber. An insufficient flow rate can lead to a lack of efficiency, or at least to a detrimentally slow reaction of the “ESP”-type trajectory control device.
To overcome this disadvantage, the invention provides a brake master cylinder comprising means for increasing the resupply flow rate across the front seal.
To this end, the invention provides a brake master cylinder of the above-described type, characterized in that the groove comprises a chamfer which extends from the bore to at least one intermediate portion of the front face of the groove with which the end of the second central lip is in contact in its rest position, and in that said end of the second central lip has a reduced cross section to facilitate the separation by tilting of the second central lip in order to cause the separation by tilting of the third outer lip, with the aim of facilitating the resupply of the front pressure chamber by the reservoir.
According to other features of the invention: the seal comprises a junction wall joining together the three lips, and said junction wall comprises a notch, arranged level with the second central lip, which is intended to facilitate the separation by tilting of the second central lip and the third outer lip; the second central lip forms a defined angle of inclination with the first inner lip and the third outer lip through which it is inclined toward the first inner lip; the end of the second central lip comprises a beveled outer face directed toward the third outer lip so as to form the reduced cross section of the second central lip; the master cylinder is of the tandem type and it comprises, in its substantially axial body, a bore inside which two axial pistons are slideably mounted, and two front seals which comprise second central lips of reduced cross section and notches, these seals being accommodated in two associated chamfered grooves.